How does a LUA Field Extractor work?

[Paul Offord <Paul.Offord () advance7 com>]

Hi,

I'm having problems accessing data in a postdissector that I am writing.  I'm porting the postdissector from LUA to C.  
I need to understand how the LUA Field Extractor works.  In the LUA script I can do the following:

  -- initialisation code
 tcp_stream_f = Field.new("tcp.stream")
  .
  .
  -- in the dissector
  this_stream = tcp_stream_f()
  tcp_stream = this_stream.value

How does this work?  How does the LUA code gain access to the tcp.stream value?

Thanks and regards...Paul

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